It is generally known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity and the radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wavelength and half wavelength antennas are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth operation, multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). Smaller packaging of state-of-the-art communications devices may not provide sufficient space for the conventional quarter and half wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing the other desirable antenna operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor. Increased gain thus requires a physically larger antenna, while communications device manufacturers and users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-frequency and/or wide bandwidth operation, allowing the communications device to access various wireless services operating within different frequency bands from a single antenna. Finally, gain is limited by the known relationship between the antenna resonant frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at the operating frequency where the effective electrical antenna length is a quarter of the operating frequency wavelength.
The known Chu-Harrington relationship relates the size and bandwidth of an antenna. Generally, as the size decreases the antenna bandwidth also decreases. But to the contrary, as the capabilities of handset communications devices expand to provide for higher data rates and the reception of bandwidth intensive information (e.g., streaming video), the antenna bandwidth must be increased.
One basic antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
The quarter-wavelength monopole antenna placed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but with the ground plane the antenna performance resembles that of a half-wavelength dipole. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. However, conventional loop antennas are too large for handset applications and do not provide multi-band operation. As the loop length increases (i.e., approaching one free-space wavelength), the maximum of the field pattern shifts from the plane of the loop to the axis of the loop. Placing the loop antenna above a ground plane generally increases its directivity.
Printed or microstrip antennas are constructed using the principles of printed circuit board techniques, where a top metallization layer overlying a dielectric substrate serves as the radiating element. These antennas are popular because of their low profile, the ease with which they can be fabricated and a relatively low fabrication cost. One such antenna is the patch antenna, comprising in stacked relation, a ground plane, a dielectric substrate, and a radiating element overlying the top substrate surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wavelength antenna, the patch antenna has a relatively poor radiation efficiency, i.e., the resistive return losses are relatively high within its operational bandwidth. Also, disadvantageously, the patch antenna exhibits a relatively narrow bandwidth. Multiple patch antennas can be stacked in parallel planes or spaced-apart in a single plane to synthesize a desired antenna radiation pattern that may not be achievable with a single patch antenna.
Given the advantageous performance of quarter and half wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency, and the antenna is operated over a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the operational frequency increases/decreases, the operational wavelength correspondingly decreases/increases. Since the antenna is designed to present a dimension that is a quarter or half wavelength at the operational frequency, when the operational frequency changes, the antenna is no longer operating at a resonant condition and antenna performance deteriorates.
As can be inferred from the above discussion of various antenna designs, each exhibits know advantages and disadvantages. The dipole antenna has a reasonably wide bandwidth and a relatively high antenna efficiency (or gain). The major drawback of the dipole, when considered for use in personal wireless communications devices, is its size. At an operational frequency of 900 MHz, the half-wave dipole comprises a linear radiator of about six inches in length. Clearly it is difficult to locate such an antenna in the small space envelope associated with today's handheld devices. By comparison, the patch antenna or the loop antenna over a ground plane present a lower profile resonant device than the dipole, but as discussed above, operate over a narrower bandwidth with a highly directional radiation pattern.
As discussed above, multi-band or wide bandwidth antenna operation is especially desirable for use with various personal or handheld communications devices. One approach to producing an antenna having multi-band capability is to design a single structure (such as a loop antenna) and rely upon the higher-order resonant frequencies of the loop structure to obtain a radiation capability in a higher frequency band. Another method employed to obtain multi-band performance uses two separate antennas, placed in proximity, with coupled inputs or feeds according to methods well known in the art. Thus each of the two separate antennas resonates at a predictable frequency to provide operation in at least two frequency bands. Notwithstanding these techniques, it remains difficult to realize an efficient antenna or antenna system that satisfies the multiband/wide bandwidth operational features in a relatively small physical volume.
The global positioning system (GPS) comprises a constellation of satellites in orbit about the earth from which geolocation information can be obtained for any location on the earth's surface. The GPS satellite signals from which the position information is derivable have a center frequency of 1.75 GHz and are circularly polarized. Of course, users and manufacturers desire minimal size antennas capable of receiving the GPS signals.
Two types of antennas that are known to provide a circularly polarized signal are the circular dipole antenna and the helix antenna. A circular dipole is illustrated in FIG. 1 as comprising four perpendicularly disposed dipole elements 2A, 2B, 2C and 2D, where elements 2A and 2B are connected to ground, element 2C is connected to a 90° or 0° phase shifter 4, and element 2D is connected to a 0° or 90° phase shifter 5 as shown. Each phase shifter 4 and 5 is connected to a feed 6 and 7, respectively. Circular polarization is achieved by feeding the elements 2C and 2D with signals having a phase difference of an odd multiple of π/2.
Each of the elements 2A, 2B, 2C, and 2D is a half wavelength in length at the operating frequency. Thus for operation at 1 GHz, each element is about 15 cm long, which is clearly too long for handset and mobile applications. The phase shifters 4 and 5 (embodied as a hybrid component or an electronic phase shifter) supply signals with the proper phase relationship, but also represent extra components for the wireless device, which in turn entails an expense and a space allotment.
A helical antenna 8 of Figure also provides a circularly polarized signal. However the antenna size, especially the height can be problematic for handset and mobile communications devices. To create a circularly polarized signal, the antenna must operate in the axial mode, where πD/λ=1, S=λ/4 and N>3. N is the number of turns in the helical antenna 8. D and S, which are indicated on FIG. 2, are the diameter of the helix and the spacing between adjacent turns. The antenna height is L=NS. Therefore, to produce a circularly polarized beam the diameter D=λ/π=0.32λ and the height H>3λ/4=0.75λ.